Capacitive sensing relies on the electrical concept of a parallel-plate capacitor, where the total capacitance is dependent on (i) the area of the two parallel plates, (ii) the distance between the two parallel plates, and (iii) the dielectric constant of the material in between the two plates, e.g.,
  C  =                    ɛ        r            d        ⁢          A      .      In many capacitive sensing applications, the human body forms one plate of the capacitor referenced to a virtual ground. Capacitive sensing is typically used to detect the presence of a human finger (or fingers) and, based on this detection, cause some action to take place. In its simplest form, the capacitive sensor replaces a mechanical switch, thus eliminating the failures typical of mechanical devices and providing a more design-friendly user interface for a product. Capacitive sensing can also be used in industrial applications as proximity detectors, e.g., to sense the presence or absence of an object on a conveyor belt. However, capacitive sensing technologies do not generally provide object recognition and identification capabilities. Instead, if a designer requires a non-contact and non-mechanical method of uniquely identifying an object, other technologies such as radiofrequency identification (RFID), near-field communication (NFC), optical scanning (e.g., bar codes, QR codes, etc.), or image recognition are utilized. Such technologies are generally cost-prohibitive and are therefore not utilized in most toy applications.
Accordingly, there is a need for a low-cost method of identifying multiple objects placed on the surface of a toy that does not require an electro-mechanical interface (e.g., physical switch or switches).